IteratorInterfaceExtensions defines a small number of extensions to the iterator interface.
This package adds a couple of extensions to the standard iterator interface in julia.
The first extension is comprised of the functions
isiterable(x) will return
false, indicating whether
x can be iterated. It is important to note that a
true return value does not indicate that one can call the
iterate method on
x, instead a consumer must call
true, and can then call
iterate on the instance that is returned by
getiterator. The proper pattern for consumer code therefore looks like this:
if isiterable(x) it = getiterator(x) for i in it # Custom code end end
This consumer pattern will work with iterators that don't opt into the extensions in this package and with iterators that have opted into the extended interface defined in this package.
There are two scenarios when a source might participate in this extended iterator interface.
The first scenario is one where a source could not implement a type-stable version of
iterate because the primary source type lacks the necessary type information. Such a source can add a method to
getiterator that returns an instance of a different type with enough type information for a type stable implementation of the core iterator interface that iterates the elements of the original source.
Second, sometimes such a source might not want to implement the
iterate method at all for its core type. If that is the case, this source can add a method to
isiterable that returns
true, even though the source does not have a
iterate method. As long as this source still implements the
getiterator function, it still complies with the extended iterator contract defined in this package.
Base.IteratorSize with an additional return value, namely
HasLengthAfterStart(). An iterator consumer that can provide an optimized implementation for iterators that know their length after the first call to the
iterate method has, can call
IteratorSize2 instead of
Base.IteratorSize. The return value will either be one of the possible return values of
HasLengthAfterStart(). If the return value is
HasLengthAfterStart(), the consumer can call
length(x, state) to obtain the number of elements the iterator will return. Here
x is the same value that
iterate was called on, and
state is the value returned by
An iterator that implements
IteratorSize2(x::MyType) = HasLengthAfterStart() must also implement
Base.IteratorSize(x::MyType) = Base.SizeUnknown().